Silviculture — defined as the theory and practice of aged for multiple uses. In a woodland made up of two or controlling forest establishment, composition, and more stands, each one may be treated differently. This growth — is important to today’s landowners who are publication emphasizes the principles of silviculture seeking more benefits than ever before from their rather than specifics of application. The discussion woodlands. These benefits include water, wildlife, and should help the reader become more familiar with the recreation, as well as timber and other wood products. silvicultural terms used in forestry. Some additional Through the use of various silvicultural practices, these references are listed at the end of the unit. Other benefits can be increased and can make the woodland publications in this series cover certain silvicultural more useful. treatments that can be used to achieve specific Nature practices its own brand of silviculture, objectives. although such natural development of the forest may take hundreds or thousands of years. Nature, of course, THE ROLE OF CUTTING IN SILVICULTURE does not consult owners about preferences. Through The same tool that can be used to destroy a forest can silvicultural practices, a manager or owner can produce a also be used to build and improve one. In the past, this woodland that meets objectives and can do it faster than tool was the axe; today, it is usually the chainsaw. can Nature. In the forest ecosystem, literally every available space is occupied by some type of vegetation. The primary SILVICULTURE FOR IMPROVED BENEFITS way that the established forest can be altered or Silviculture involves developing a plan for influencing controlled is by killing trees and other plants. In most the forest to grow in a certain way in order to achieve cases trees are killed by cutting. If the trees have no certain goals. The range of silvicultural treatments commercial value, it is cheaper to kill them by applying possible depends on the biological requirements and a chemical herbicide or by girdling them. Girdling, growth characteristics of the tree species present, along which involves the removal or killing of a ring of bark with the characteristics of the site where they are around the tree to block the flow of food from the crown growing — including all of the physical, chemical, to the roots, causes the tree to starve and die. climatic, soil, and biological features of the area that Most silvicultural treatments have the objective of influence tree development and growth. Trees require promoting the growth of desirable species while water, nutrients, carbon dioxide, and sunlight to grow. discouraging less desirable species. In young forests, the Silvicultural practices regulate the availability of water, desirable trees are nurtured by removing the undesirable nutrients, and sunlight to selected trees. ones. In mature or overmature forests, old trees are A number of silvicultural treatments often can be used removed in order to provide room for new trees. From effectively for a given woodland. The treatment chosen the silvicultural perspective, cutting trees is the primary depends on the objectives and desires of the woodland method of establishing and tending forest stands to meet owner. It is important to realize that no single a woodland owner’s objectives. Cutting trees to obtain silvicultural treatment or scheme is best for all wood products and income may be of secondary woodlands and all owners. As long as biological con- importance. straints are respected, there often are many right ways to treat a stand silviculturally. Furthermore, primarily for INTERMEDIATE CUTTINGS sociological and economic reasons, no two landowners Any cuttings or treatments applied to established are likely to seek exactly the same set of benefits from immature forests for the purpose of improving them are their woodland. A woodland can be managed for a single referred to as intermediate cuttings and may be dominant use, or it may be man- undertaken for any of the following reasons:

• To remove poor quality trees. They have medium-size crowns and, along with the • To remove weed tree species. dominants, they form the upper canopy of the forest. • To thin the stand and increase the growth rate of Intermediate trees are shorter than those of the preceding residual trees. two classes. Their crowns extend into the upper canopy • To free an understory of seedlings and saplings from layer; however, they receive very little light from above suppression by an overstory of larger, inferior quality and no light from the side. Suppressed or overtopped trees. trees are totally below the main canopy. They are the • To remove insect- or disease-infested trees. smallest trees and receive no direct sunlight. • To salvage timber damaged by insect, disease, or fire. Dominant and codominant trees grow fastest because they have large crowns that can absorb the sunlight Stand development needed for growth. Intermediate and suppressed trees In order to understand the biological basis for most grow more slowly and usually will not grow much faster intermediate treatments, it is helpful to know how a even if the forest is thinned to give them more sunlight. natural forest stand develops. Many forest stands in Such trees may be genetically inferior specimens, trees Pennsylvania originated during the early part of this that naturally grow slowly, or damaged trees. It is century, after repeated harvests for timber and fuel- important to note that these smaller trees in the lower wood. These harvests were in many cases followed by canopy are generally the same age as their faster- severe fires. Stands that originate over a relatively short growing, more vigorous associates. period of time are referred to as even-aged, that is, all the trees in such stands are of roughly the same age. During the early stages of stand development, following heavy cutting or other disturbances, thousands or tens of thousands of small trees may be growing on an acre. As the trees get larger, they begin to compete with one another for the limited amounts of light, moisture, and nutrients. Natural selection occurs and the number of trees gradually decreases (Table 1). At maturity, only 80 to 100 trees may be growing on an acre. The trees that die during this process eventually fall to the ground and slowly decompose. Crown position Growth in height is the most critical factor determining the survival of forest trees. As trees begin to compete with one another, the stronger trees forge ahead, grow taller, and develop large tops or crowns. Weaker trees are crowded by their competitors, and their crowns Shade tolerance become misshapen and restricted in size. This Tree species vary in their shade tolerance — their ability competition between trees for places in the forest canopy is described by differentiation of trees into crown to grow and reproduce in the shade of other trees. classes. Tolerant trees are those capable of growing in heavy Trees in a forest stand can be classified according to shade. Examples of tolerant trees are beech, sugar maple, the position and size of their crowns in relationship to and hemlock. Other species such as black cherry, paper the others in the stand (see figure). Dominant trees are birch, or tulip poplar require full sunlight in order to the largest trees in a stand. They have large, full crowns, grow properly; they are referred to as intolerant. Species and receive full sunlight from above and partial light such as red and white oak, red maple, and ash can grow from the sides. Codominant trees receive full light from in moderate shade and are said to have intermediate above but relatively little from the sides. tolerance.

Table 1. Average size and number of trees per acre for a typical unmanaged, even-aged, mixed-oak forest at different ages. Age (years) 10 20 30 40 50 60 70 80 90 100 Trees per acre 3,200 1,800 930 505 342 262 215 187 150 90 Average dbh (inches) 1.2 2.4 4.0 5.9 7.5 8.8 9.9 10.7 12.2 14.8

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Trees that are fittest from the standpoint of natural cutting, or salvage cutting, to describe different selection and thus destined to become dominant and intermediate treatments referred to collectively as TSI codominant in natural stands, are not always the best (Timber Stand Improvement). The principle underlying trees from the woodland owner’s point of view. all these treatments is the same: undesirable trees are cut Intermediate cuttings allow the woodland owner to and removed in order to promote the growth of the select which trees will remain and which will die. The desirable trees. A woodland owner’s objectives number of trees per acre is then reduced as it would be in determine which trees are desirable (trees to leave) and a natural stand, but trees that are desirable to the owner which ones are undesirable (trees to cut). Terms such as are favored while those that are less desirable are WSI (Wildlife Stand Improvement) or ASI (Aesthetic removed (Table 2). Stand Improvement) might better reflect the objectives of some owners. Table 2. Distribution of trees by diameter class before and after an improvement cut in a 70-year-old even-aged stand of oak-hickory. Dbh class Trees per acre SILVICULTURAL HARVESTING (inches) Before cut After cuts Tree In every forest stand, at some point all or some of the 4 12 12 0 trees reach maturity. As a tree becomes mature, height 6 24 8 16 and diameter growth decrease and eventually stop. More 8 12 2 10 branches die, wounds heal more slowly, and the tree’s 10 24 10 14 ability to resist damage by insects and disease decreases 12 30 10 20 considerably. Eventually, the tree is harvested by natural 14 36 30 6 16 20 14 6 forces. Certain tree species reach maturity at a relatively 18 12 6 6 early age; others continue to grow for a much longer 20 2 2 0 period of time. Some birches and aspen, for example, 22 4 4 0 reach maturity at 50 to 60 years while many species of oak reach maturity at 150 to over 200 years. Regulating growth An optimum size and age to which trees should grow The total amount of growth an acre of forest is capable can be determined for specific situations. These of supporting is fixed and dependent on site quality (the optimum conditions depend on the growth patterns of amount of light, water, and nutrients available). the species involved, site quality, economic markets Intermediate cuts generally cannot influence the total available for wood products, and the objectives of the amount of growth that occurs, but they can influence woodland owner. The period of years required to grow how the growth is distributed in the forest stand. Growth trees to these specified conditions of either economic or in unmanaged stands is applied to trees of both good and biological maturity is known as the rotation. The poor quality; all trees grow and their volume increases at rotation for fast-growing, short-lived species such as a relatively slow rate. Through intermediate cuttings, aspen or birch used for pulp may be as short as 25 to 30 individual trees are selectively removed. Residual trees years. Slower-growing hardwoods, such as oaks, used expand their crowns and grow faster by using the primarily for sawtimber, commonly have rotations of 70 additional moisture, light, and nutrients made available to 100 years. when their competitors are removed. The total amount of growth per acre remains the same, but it is now Regeneration distributed on fewer and faster-growing trees. When a large number of trees in a stand reach financial Redistributing the growth on fewer trees results in a or biological maturity, it is time for the woodland owner stand occupied by larger, more vigorous trees. These to think about harvesting the old trees and replacing trees are often more resistant to attack by insects and them with others of a new generation. Regeneration or disease, and they produce more mast for wildlife. A reproduction cuttings are made for the purpose of great economic benefit also results from such treatments. removing old trees as efficiently as possible and creating A single 20-inch dbh tree with a merchantable height of environmental conditions favorable for the establishment 40 feet contains as many board feet as six 10-inch dbh of a new crop. Concern for the immediate regeneration trees of the same height. Other benefits of thinning are of new trees is the most significant difference between the salvage and use of wood that would normally be lost silviculture and exploitative logging, and for this reason to natural mortality. Intermediate cuts remove trees these treatments are referred to as reproduction cuts before natural suppression occurs; a series of rather than harvest cuts. improvement cuttings increases the yield of a forest by In the Northeast, foresters generally rely on forests to 20 to 35 percent. regenerate themselves prior to or following cutting, Foresters use a number of terms, such as release either through natural seeding or by sprouting. In some cutting, liberation cutting, thinning, improvement cases, tree seedlings are established in a stand prior to 3 cutting. Their presence is referred to as advance become established. All trees larger than 2 inches dbh regeneration. In almost all cases, woody or herbaceous usually are cut, including culls and trees of low quality. vegetation quickly occupies any site following a The silvicultural clearcut should not be confused with disturbance to the existing mature trees. Trees that seed what is often referred to as the commercial clear-cut, naturally after an unplanned logging operation, or which is a nonsilvicultural harvest. Commercial similar disturbances, may not be the species that the clearcutting removes only merchantable trees, leaving owner considers valuable. If the goal is to ensure prompt behind small, damaged, and poor-quality trees. regeneration of a certain species of tree through natural Clearcutting allows maximum sunlight to reach the seeding, then the reproduction cut is probably the most soil surface. Intensive logging activity disturbs the forest difficult silvicultural task a forester can attempt. The litter layer and exposes mineral soil. Application of this success or failure of this treatment will be evident in the method assumes that an ample supply of seedlings or vegetation for 50 to 100 years or more. seeds are already in the ground or that seed will be blown in from adjacent stands. In the latter case, size and Planting shape of the clearcut must be restricted by the distance A forest stand established by artificial planting of tree seed can be carried by wind. seedlings is referred to as a plantation. In common Clearcutting favors reproduction and growth of tree forestry practice, trees are planted as one- to three-year- species that thrive in full sunlight. These species include old bare-rooted seedlings. Conifers are planted in much all the more valuable tree species in both the Allegheny greater numbers than are hardwoods. In Pennsylvania, Hardwood and the Oak-Hickory forest types (including planting is usually carried out only where natural black cherry, white ash, red oak, white oak, and yellow regeneration has failed or where open land is converted poplar). In the absence of competition, young trees are to forest land. unencumbered and grow quickly. In Pennsylvania, most valuable tree species respond well to clearcutting only if REPRODUCTION METIIODS they are present as advance regeneration prior to Choosing the appropriate reproduction method is a cutting. complex process involving ecological, economic, and The clearcutting method should not be used where sociological considerations. Here the forester’s skill and there is insufficient advance regeneration, where ferns perceptions are used to make trade-offs between costs and grasses occupy over one-third of the site, where the and aesthetics, immediate and future needs, productivity water table has a potential to rise close to the surface and preservation. Some of the variables a forester and after harvesting, where shallow soils occur over rock, or woodland owner need to keep in mind when selecting a where continuous forest cover is needed to prevent method are: stream siltation or increases in stream temperature. • Biological requirements of desirable and undesirable From the administrative and logistical viewpoint, tree species. clearcutting is the simplest reproduction method to • Characteristics of the site, including soil, topography, implement. Since residual trees are not left, less moisture, and microclimate. supervision of cutting is required and fewer constraints • Economic markets for wood products. are imposed on the harvesting equipment and operation. • Impacts of cutting on wildlife, water, recreation, and Many people find clearcutting to be the least desirable aesthetics. reproduction method for aesthetic reasons. For the first • Methods of controlling damaging insects, diseases, two or three years after the cut, no conspicuous growth and fires. is evident. Logging debris and slash may contribute to • Limitations of available harvesting equipment. the desolate appearance of the site. Close scrutiny, The three methods commonly used in regenerating or however, often reveals a new stand of young seedlings. reproducing a stand of trees in Pennsylvania are Soon a new stand of sapling-sized trees will occupy the clearcutting, shelterwood, and selection. Each involves site and make the area more visibly appealing. The removal of the mature overstory in a manner that initial visual impact of clearcutting can be tempered by encourages seedlings of desirable species to regenerate strategically regulating the size, shape, and location of the stand naturally. These methods differ in the the cut areas so that they blend in naturally with the combinations of light, temperature, and moisture created landscape. on the forest floor. The clearcutting and the shelterwood Many kinds of forests are composed of natural methods are used to establish even-aged stands, and the vegetation that resulted from fire or other destructive selection method is used to create or maintain uneven- disturbances. Clearcutting merely mimics these natural aged stands. An uneven-aged stand contains at least disturbances. Most of today’s Pennsylvania forests re- three different age classes of trees. sulted from what amounted to clearcutting at or near the turn of the century. Clearcutting Clearcutting is a single harvest of all trees in a stand with the expectation that a new even-aged stand will 4

Shelterwood selected for harvest on the basis of their economic or When the shelterwood method is used, a new stand is biological maturity and their relative contribution to the established under the shelter of older trees. The old stand long-term productivity and value to the stand. is generally removed in a series of two or three cuttings Regeneration cuts and intermediate or improvement cuts made at intervals of five or ten years. The new forest is are carried out simultaneously, and new seedlings or considered to be even-aged. sprouts grow up in the spaces created. Smaller and low In the first cut, trees are selectively removed value trees are also cut to ensure proper spacing and throughout the stand. This cut increases vigor, stimulates future growth of desirable species that have not yet seed production of the remaining trees, allows some reached maturity. Both before and after cutting, the stand sunlight to reach the forest floor, and prepares the site should include trees representing a wide range of for the seed by stirring the forest floor and exposing diameter classes (Table 5). mineral soil. New seedlings become established under the partial shade of the residual trees. Once the seedlings Table 3. Simplified examples showing distribution of trees by are well established, the remaining overstory trees are dbh class, before and after harvest, using the selection method in an uneven-aged northern hardwood stand. removed in one or two more cuts to free the seedlings from competition. Trees per acre The trees left after the first cut serve as sources of Dbh class seed and shade for the new crop. This method is best (inches) Before harvest After harvest Trees cut suited for intermediate and tolerant tree species that do 6 65 54 11 not compete well with other vegetation when growing in 8 52 34 18 direct sunlight. By regulating the severity of the first cut, 10 31 21 10 12 28 13 15 this system can be adapted to all but the most intolerant 14 12 8 4 species. 16 6 5 1 The shelterwood method should be used when 18 3 3 — 20 3 2 1 advance regeneration of desirable species is not adequate 22 2 1 1 to permit clearcutting. Regeneration may be inadequate 24 1 — 1 for several reasons. The stand may be so dense that seedlings do not have adequate sunlight to survive. Ferns and other competing vegetation may grow in sufficient The periods between selection cuttings are referred to quantities to kill seedlings; browsing deer may destroy as cutting cycles. To ensure that a harvest is large young trees. Competing vegetation can be controlled enough to be worthwhile, cutting cycles generally range with an appropriate herbicide. If the deer population is from five to twenty years. excessive, it may be necessary to kill the deer, to fence Because relatively few trees are harvested at one time them out, or to stimulate seedling growth with fertilizer. and because the forest floor is generally shaded, this Since the new stand is already established in the method favors tree species such as sugar maple, beech, shelterwood before the old stand is completely removed, and hemlock that can reproduce and grow in low-light regeneration failures seldom occur, and the forest does levels. One of the disadvantages of this method is that not have the devastated, barren appearance of clearcut certain high-value intolerant and intermediate species are areas. The gradual removal of the old stand also provides difficult or impossible to regenerate. maximum yield because mature trees are removed in the The selection method is best applied to existing first cut. Vigorously growing trees can continue valuable uneven-aged stands. In Pennsylvania such stands are production for a number of years while they serve as a relatively rare because of past disturbances. The source of seed. selection system may be applied to an even-aged stand One disadvantage of the shelterwood method is the in order to create an uneven-aged stand. This, however, need to bring logging equipment into the stand at is a very difficult process. It may take three or four frequent intervals. This is more costly than the one-time cuttings over a period of twenty to forty years to obtain clearcut, and residual trees or new seedlings may be the proper distribution of size classes. damaged or destroyed when other trees are being felled The selection method has the least visual impact of or transported. all the reproduction methods. Trees are always present on the site, and cutting activity is generally not Selection noticeable to the passerby. However, administrative and The goal of the selection method is to create or maintain supervising costs are very high. Proper application of the stands that contain trees representing a mixture of ages selection method requires the repeated and frequent and size classes (uneven-aged). In theory, this method attention of a forester. The stand has to be inspected, produces stands with equal areas occupied by mature cruised, and marked before each cutting in order to trees (sawtimber), poles, saplings, and seedlings. ensure proper distribution of size and age classes. Selection cutting involves the removal of individual or Logging operations occur at frequent intervals and must small groups of trees at frequent intervals. Trees are 5 be closely supervised in order to ensure that residual ADDITIONAL READINGS trees are not damaged. Beattie, N., C. Thompson, and L. Levine. 1983. Working with Your Woodland: A Landowner Guide. Nonsilvicultural Harvests University Press of New England, Hanover, NH. A number of commonly used harvesting methods should 310 pp. ($9.95) be avoided and not confused with the silvicultural Daniels, T. W., J. A. Helms, and F. S. Baker. 1979. procedures just described. Several different procedures Principles of Silviculture. McGraw-Hill, New York, and names are used, but most of these nonsilvicultural NY. 500 pp. ($25.00) harvesting methods are simply variations of high Forbes, R. D. 1976. Woodlands for Profit and Pleasure. grading, which greatly decreases the long-term American Forestry Association, Washington, DC. productivity of any woodland. Such harvest procedures 253 pp. ($6.25) neglect the silvicultural needs of the stand and the Minckler, L. 1980. Woodland Ecology: Environmental environmental needs for proper regeneration. Forestry for the Small Owner. Syracuse Univ. In nonsilvicultural harvest systems, the selective Press, Syracuse, NY. 241 pp. ($9.95) removal of larger, high-value trees leaves behind only Society of American Foresters. 1981. Choices in the smaller trees or other low-value trees of inferior Silviculture for American Forests. Washington, DC. species or poor quality. The diameter-limit cut is the 80 pp. ($4.25) most frequent type of nonsilvicultural harvest used in Smith, D. N. 1962. The Practice of Silviculture, John Pennsylvania. When using this procedure, a timber Wiley & Sons, Inc., New York, NY. 578 pp. buyer contracts to harvest all timber larger than a certain ($29.00) diameter — usually 12 or 14 inches. The ease with which this method can be applied and the resulting high, one- time economic return make it attractive in spite of its long-term drawbacks. Diameter-limit cutting is sometimes defended as a means of removing the larger, older trees and leaving the smaller, younger trees to grow. however, it is commonly applied to even-aged stands where the smaller trees are Prepared by James R. Grace, Assistant Professor, Forest Resources not younger but rather the same age as the trees cut. Extension, School of Forest Resources. Frequently, the smaller trees are slower-growing, Issued in furtherance of Cooperative Extension Work, Acts of Congress damaged, or inferior and less vigorous species. Many of May 8 and June 30,1914, in cooperation with the U.S. Department of these trees were in the intermediate and suppressed Agriculture and the Pennsylvania Legislature. L.F. Hood, Director of Cooperative Extension, The Pennsylvania State University. crown classes in the original stand, and they may be incapable of responding to release once the dominant and codominant trees are removed. This publication is available in alternative media on request.

After one or more diameter-limit cuttings, stands are dominated by low-value, poor-quality trees. Future The Pennsylvania State University is committed to the policy that all persons shall have equal access to programs, facilities, admission, and employment productivity and economic yield decrease substantially. without regard to personal characteristics not related to ability, performance, or qualifications as determined by University policy or by state or federal authorities. The Pennsylvania State University does not discriminate against any person because of age, ancestry, color, disability or handicap, national origin, race, religious creed, sex, sexual orientation, or veteran status. Direct all inquiries regarding the nondiscrimination policy to the Affirmative Action Director, The Pennsylvania State University, 201 Willard Building, University Park, PA 16802-2801; tel. (814) 863-0471; TDD (814) 865-3175. File No. IVI1h R3M994 U.Ed. 86-254

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